The invention relates to a brake disc for a vehicle disc brake, particularly for passenger cars, and especially to a ventilated brake disc having a friction ring. In the case of a ventilated brake disc having a double-walled friction ring, the friction ring is supported by a brake disc chamber made of a specifically light material, which has a central flat hub section perpendicular to the axis of rotation of the brake disc, as well as a so-called support wall section which is connected thereto, extends radially outward and in some regions has a truncated circular cone shape, with whose free edge region the possibly internally ventilated friction ring, which consists of a different material, is connected by way of connection elements. Concerning the state of the art, reference is made, for example, to German Patent document DE 10 2006 043 945 A1.
So-called built-up motor vehicle brake discs having a friction ring made of a first material, such as gray cast iron or a special steel, which interacts with brake pad friction linings during operation, and having a so-called brake disc chamber made of a different material, such as a light metal alloy, which supports this friction ring, are known in many different developments. On motor vehicles, particularly passenger cars, it is endeavored to design all components, and particularly the unsprung masses, which include the brake discs normally arranged directly on or within the wheel(s), to be as light as possible. The brake disc chamber is therefore manufactured of a material of low specific weight, thus, particularly of an aluminum alloy. Accordingly, the friction ring can also consist of a specifically extremely light material. However, because of the interaction with the friction lining, no conventional aluminum alloy can be used for this purpose, so that, even when suitable specifically light friction ring materials are used, a brake disc has to be assembled of a friction ring, that is manufactured of a first, specifically suitable material, and of a brake disc chamber, that is manufactured of a second, different specifically suitable material. However, in the case of such a brake disc assembled from different materials, which, or especially whose friction ring, as known, may significantly heat up during operation, the different thermal expansion behavior of the various materials may result in problems, for example, in the form of a shielding known to the person skilled in the art, but at least in undesirable tensions in the brake disc. It should be explicitly mentioned at this point that, as an alternative, the friction ring may also be shaped of a material that, compared with the material of the brake disc chamber, is specifically heavy or heavier, which leads to the same problems.
There is therefore needed a functionally reliable brake disc which is characterized by a further weight reduction.
This need is met by providing a brake disc wherein the ratio of the height of the friction ring measured in the radial direction to the diameter of the brake disc is less than 0.12 with respect to the amount, and that the connection between the brake disc chamber and the friction ring with the prestressed support wall section is established such that the edge region of the brake disc chamber acts in an outwardly pressing manner in the radial direction on the friction ring directly or indirectly by way of the connection elements when the friction ring has not been heated by a braking operation.
Two mutually functionally related measures are provided, specifically a geometrical design configuration, whose implementation becomes possible only on the basis of the second mentioned measure, specifically by the quasi-prestressed brake disc chamber, because only that can achieve a functionally reliable shaping of the brake disc as a whole. Concretely, it is first provided to, viewed in the radial direction, design the friction ring narrower than is customary; i.e. to provide a friction ring with a lower height (measured in the radial direction) than previously customary. In any case, if the material of the friction ring is specifically heavier than the material of the brake disc chamber, a reduction of the friction ring height (measured in the radial direction) causes an especially intensive reduction of the weight of the brake disc, particularly because the friction ring is situated farther to the outside in the radial direction than the brake disc chamber.
In the present case, the reduced friction ring height is defined by the ratio of the friction ring height to the total diameter of the brake disc. In the case of known brake discs of passenger cars in use today, the above-mentioned ratio is in the order of magnitude of between 0.14 and 0.17. In the present case, a brake disc is now provided that has a friction ring whose height is maximally 0.12 times the entire brake disc diameter.
By means of a brake disc friction ring dimensioned as disclosed above, if calipers are used that are currently customary and interact with the friction ring, in connection with slightly narrower brake pads, braking torques can be reached that are normal in the case of passenger cars; particularly if the diameter of the brake disc is increased slightly in comparison to the design currently customary, because then, while the pressing force of the brake pads onto the friction ring remains the same, a larger lever arm will be available for the resulting braking torque.
However, a friction ring that is narrower than previously customary friction rings, i.e. it is not as high, during the operation, i.e. during high braking operations, may heat up more than a conventionally dimensioned friction ring of a built-up brake disc having a brake disc chamber carrying the friction ring. Even though the thermal expansion coefficient of a specifically heavy friction ring material, such as gray cast iron or a special steel, may possibly be (significantly) lower than the thermal expansion coefficient of the material of the brake disc chamber, which preferably may be an aluminum alloy, because of the fact that the temperature of the friction ring becomes significantly higher during braking than that of the brake disc chamber, a different thermal expansion of these two component parts of the brake disc can be determined. The same may also apply to the friction ring when certain specifically extremely light materials are used; i.e. also when using such materials (which are considerably more expensive compared with gray cast iron or special steel, and can therefore be used economically only for a few applications), the problem may occur that the shielding known to the person skilled in the art has to be prevented. In addition, the above-mentioned specifically extremely light friction ring material may be an aluminum-saturated silicon-carbide matrix or a carbon fiber silicon carbide, which are both very suitable as friction partners for brake pad friction linings of disc brakes.
So that the different thermal expansion mentioned in the preceding paragraph, i.e., the thermal expansion of the friction ring that is higher than the thermal expansion of the brake disc chamber, will not result in problems, particularly in the connection region between the friction ring and the brake disc chamber or in the above-mentioned shielding, the brake disc chamber is built together or connected in a prestressed manner with the friction ring. This prestressing will have the effect that, when the friction ring is not heated by a braking operation, the edge region of the brake disc chamber acts in an outwardly pressing manner in the radial direction upon the friction ring directly or indirectly by way of the connection elements, and this prestressing can easily be represented or is represented particularly by the shaping of the brake disc chamber, specifically by the truncated-cone-shaped support wall section and/or its edge region.
By means of the characteristics explained above, it becomes possible, deviating from the brake disc indicated in DE 10 2006 043 945 A1, to provide connection elements in the form of rivets or screws between the brake disc chamber and the friction ring, whose longitudinal axis extends parallel to the axis of rotation of the brake disc. Such connection elements can be inserted more easily than the pins provided in the above-mentioned state of the art which extend in the radial direction.
If the support wall section has a plurality of openings, the weight of the brake disc chamber can advantageously also be kept low. In this case, the required stability can, for example, be achieved in that the support wall section is designed, at least in regions, in the form of a framework. Furthermore, again in view of the different thermal expansions of the friction ring and the brake disc chamber, the edge region of the support wall section can, in each case, have an at least slightly elastic construction between connection elements that are mutually adjacent in the circumferential direction, which can be implemented in a particularly intensive manner if the edge region is constructed in the form of individual mutually separated segments. A construction of the edge region of the brake disc chamber which may, for example, be in the form of a “swan-neck”, and is at least slightly elastic viewed in the radial direction, can be used for the same purpose as well as for generating a desired prestressing, by which the edge region of the brake disc chamber can act upon the friction ring while pressing outward in the radial direction.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.